1. Field of the Invention
The present invention relates to a liquid crystal display device, and more particularly to a liquid crystal display device having optical sensors embedded in a liquid crystal panel to sense a touch and capable of preventing crosstalk by changing positions of read-out lines to improve touch sensitivity and a method for manufacturing the same.
2. Discussion of the Related Art
As an information-oriented society has been developed, a display field has been rapidly developed to visually express electrical data signals, and various flat display devices having excellent performances, such as thin profile, light weight, and low power consumption, have been developed and rapidly replaced a conventional cathode ray tube (CRT).
The flat display devices include a liquid crystal display device (LCD), a plasma display panel device (PDP), a field emission display device (FED), an electro luminescence display device (ELD) and the like. All the above-mentioned flat display devices essentially include a flat display panel to display an image. The flat display panel includes a pair of transparent insulating substrates, which are bonded to each other while an intrinsic luminescent or polarizing material layer is interposed therebetween.
Among those flat display devices, a liquid crystal display device displays an image by adjusting the light transmittance of liquid crystal using an electric field. Accordingly, the liquid crystal display device includes a display panel having liquid crystal cells, a hacklight unit for illuminating light onto the display panel, and driving circuits for driving the liquid crystal cells.
The display panel is configured such that a plurality of gate lines and a plurality of data lines intersect each other to define a plurality of unit pixel regions. In each of the pixel regions, there are provided a thin film transistor array substrate and a color filter substrate facing each other, a spacer positioned to maintain da specific cell gap between the two substrates, and liquid crystal filled in the cell gap.
The thin film transistor array substrate includes the gate lines and the data lines, thin film transistors serving as switching elements and formed at the intersections of the gate lines and the data lines, pixel electrodes formed in liquid crystal cell units and connected to the thin film transistors, and an alignment layer coated thereon. The gate lines and the data lines receive signals from the driving circuits through pads, respectively.
The thin film transistors supply pixel voltage signals supplied to the data lines to the pixel electrodes in response to scan signals supplied to the gate lines.
The color filter array substrate includes color filters formed in liquid crystal cell units, a black matrix provided to divide the color filters from each other and reflect external light, a common electrode for supplying a reference voltage to the liquid crystal cells in common, and an alignment layer coated thereon.
The thin film transistor array substrate and the color filter array substrate, which are separately manufactured, are aligned, and then are bonded to each other. Thereafter, liquid crystal is injected into a gap between the two substrates and the gap is sealed.
In the liquid crystal display device manufactured as described above, recently, optical sensors are formed inside the display panel to control a backlight unit according to the brightness of external light A touch panel, which has been attached to the outside of the display panel to cause an increase in volume, is intended to be installed inside the display panel.
Hereinafter, a conventional liquid crystal display device including optical sensors will be described with reference to the accompanying drawings.
FIG. 1 illustrates a cross-sectional view showing a conventional liquid crystal display device including optical sensors.
As shown in FIG. 1, the conventional liquid crystal display device including optical sensors includes a liquid crystal panel 10 having optical sensors 7, a plurality of optical films 11 positioned under the liquid crystal panel 10, a light source such as a lamp provided under the optical films 11, a backlight unit 20 for coupling and supporting the optical films 11 and the liquid crystal panel 10 disposed thereon, and a casing 30 for covering a side portion of the liquid crystal panel 10 excluding an upper surface thereof and the backlight unit 20.
In this case, the liquid crystal panel 10 includes a thin film transistor substrate 6 having the optical sensor 7 and a color filter substrate 5 facing each other. A liquid crystal layer (not shown) is formed between the color filter substrate 5 and the thin film transistor substrate 6 to perform a display function according to application of voltage.
Further, a passivation film 8 is positioned on the liquid crystal panel 10 to prevent damage of the liquid crystal panel 10 due to contact of the fingers.
The operation of the conventional liquid crystal display device including optical sensors is explained.
The optical sensors 7 of the thin film transistor substrate 6 detect illuminance of the backlight unit 20 and external light and detect the light reflected by a finger 1 or a shadowed portion to read touch coordinates when a touch action occurs, thereby sensing a touch. That is, in a portion where a touch has been made, the external light is shielded by the finger 1 and a portion of light emitted from the backlight unit 20 is reflected from the surface of the finger 1, thereby sensing a touch by a light emission amount.
In this case, output voltages of the optical sensors 7 are represented in a gray scale. One optical sensor provides one pixel value as in a picture. The touch coordinates are perceived by applying such an algorithm.
FIG. 2 illustrates a circuit diagram showing a conventional liquid crystal display device. FIG. 3 schematically shows a parasitic capacitance problem generated between a read-out line and a neighboring data line of FIG. 2.
As shown in FIGS. 2 and 3, in a general liquid crystal display device, a plurality of gate lines 41 and data lines 42a, 42b and 42c intersect each other to define pixel regions. Thin film transistors Tpixel are provided at intersecting portions of the gate lines 41 and the data lines 42a, 42b and 42c. A driving voltage line (VDRV) 43 and a storage voltage line (VSTO) 45 are spaced from each of the gate lines 41 in parallel. Further, pixel electrodes are formed in the pixel regions.
In this configuration, the data lines includes first to third data lines 42a, 42b and 42c of B, R and G colors, respectively. A read-out line 51 is formed adjacent to the second data line (red data line) 42b. The read-out line 51 is positioned between the outside of the pixel electrode of the neighboring first data line (blue data line) 42a and the second data line (red data line) 42b. 
Each of the optical sensors 7 of FIG. 1 includes an output transistor Ta having a gate electrode connected to the previous gate line 41 and a drain electrode connected to the read-out line 51, a capacitor Cs formed between a source electrode of the output transistor Ta and the storage voltage line 45, and a sensing transistor Ts having a drain electrode connected to the source electrode of the output transistor Ta, a source electrode connected to the driving voltage line (VDRV) 43 and a gate electrode connected to the storage voltage line (VSTO) 45.
The operation of the optical sensor is explained. That is, a driving voltage of 12V is applied to the source electrode of the sensing transistor Ts through the driving voltage line 43, and a voltage of OV is applied to the gate electrode of the sensing transistor Ts through the storage voltage line 45. When a certain amount of light is sensed in an active layer of the sensing transistor Ts, photo current is generated to flow from the source electrode to the drain electrode of the sensing transistor Ts through a channel according to the sensed light amount. The photo current flows to the capacitor Cs through the drain electrode of the sensing transistor Ts. Accordingly, charges generated by the photo current are accumulated in the capacitor Cs by the driving voltage line 43 and the storage voltage line 45. The charges accumulated in the capacitor Cs flow to the read-out line 51 through the output transistor Ta to be detected by a detection unit (read out IC) connected to the read-out line 51. It is determined whether a touch is made based on the value of the photo current.
That is, a signal detected in the detection unit connected to the read-out line 51 is changed according to the light amount sensed in the sensing transistor Ts. Accordingly, it is possible to sense an image, for example, a document, image scan, and touch input. The sensed image may be transmitted to a controller or the like or displayed on the screen of the liquid crystal display panel under the control of the user.
As shown in FIG. 3, a large parasitic capacitance 46 is formed between the read-out line 51 and the second data line (red data line) 42b, adjacent to each other and formed on the same layer, due to the optical sensor.
Further, a second parasitic capacitance is generated in the first data line (blue data line) 42a formed in the same pixel region as the read-out line 51. The second parasitic capacitance is smaller than the parasitic capacitance 46, but may influence the display.
Resultantly, vertical crosstalk may occur due to the parasitic capacitance and the second parasitic capacitance.
The vertical crosstalk is one of factors causing a malfunction in application of a touch algorithm. Particularly, in a normally white LCD in which a voltage is applied to red pixels and no voltage is applied to green and blue pixels, crosstalk of display increases.
An increase in crosstalk is caused by the position of the sensors. Recently, the sensors are positioned only at blue pixels. The sensors are provided at the nearest positions to the red data lines on the mask. Accordingly, a parasitic capacitance value between the read-out line and the red data line is the largest value, and there is variation in the sensing screen according to colors.
The conventional liquid crystal display device has the following problems.
When optical sensors are provided externally, it is impossible to manufacture a slim liquid crystal display device. In attachment of parts for connecting optical sensors to an internal liquid crystal panel, a large amount of parts and time are required.
Accordingly, when optical sensors are provided internally, the optical sensors are generally positioned at blue pixel regions. Particularly, the read-out lines for detecting a voltage or current value sensed by the optical sensors should be formed in parallel and adjacent to the data lines for displaying a red color, which are the nearest to the blue pixel regions. Thus, severe vertical crosstalk occurs at these positions due to generation of parasitic capacitance.
Further, the parasitic capacitance also influences the blue pixel regions at which the optical sensors are positioned.
Further, the amount of charges increases in the capacitors included in the optical sensors due to sensing of touch and the charges are transmitted to the storage voltage lines, thereby causing horizontal crosstalk due to distortion of a phase voltage applied to storage voltage lines.